Chronic pancreatitis (CP) is characterized by inflammation, fibrosis, and loss of pancreatic cells. CP can lead to sufficient tissue destruction and result in exocrine and endocrine insufficiency as well as difficult to treat chronic abdominal pain. The management of CP is challenging as there are no effective methods that can stop progression or reverse the disease. In addition, CP is a risk factor for the development of pancreatic cancer. Absence of progress in CP therapy in part is due to lack of understanding in mechanisms that potentially can either reverse or halt the disease. Recent in vitro and in vivo studies have shown objectively the role of activated pancreatic stellate cells (PSCs) in fibrogenesis in CP. PSCs play a central role in disease progression by regulating the synthesis and degradation of extracellular matrix proteins. Activation of PSCs is increased by cytokines from injured acinar cell and infiltrating leukocytes (e.g. macrophages). However, the mechanisms by which macrophages trigger and sustain the fibrotic processes are not fully understood. Our initial studies show that subset of macrophages (known as M2) are dominant in CP as compared to acute pancreatitis (AP), which is dominated by a different subtype of macrophages (M1). We hypothesize that unlike M1, M2 macrophages promote pancreatic fibrosis through sustained activation of PSCs, and in turn the activated PSCs enhance macrophage polarization towards M2 and propose mechanisms involved in macrophage-PSCs interaction to promote progression of CP. To achieve this goal in aim 1, we use animal models of acute and chronic pancreatitis in order to characterize immune responses associated with CP. Under aim 2, we will determine the role and source of macrophage subpopulation in CP. Specifically, we propose to test and study subtypes of macrophages and cytokines that are essential in the development of CP. Under aim 3, we will identify mechanisms via which macrophages affect PSC function and how PSCs in turn affect macrophage polarization and behavior using various co-culture systems and in vivo using transgenic animals that allow testing of pertinent pathways in the immune responses. When possible human PSCs (isolated from surgical specimens) and monocyte/macrophages will be used to confirm and identify pathways involved. Our proposed studies are novel because they focus on the immune responses associated with CP, macrophage plasticity, and have the potential to shift paradigm in the field by proposing mechanisms that can either halt or reverse CP, and thus alter the natural course of the disease. In addition to the gained understanding of immune mechanisms that mediate and/or allow progression of CP, the potential impact of this project is of great clinical significance, as our studies may lead to development of novel therapies that can change how we manage patients with CP.